Sterilization of medical devices may be provided by several means. Two common means are ethylene oxide sterilization (EO) and sterilization by exposure to ionizing radiation. However, exposure of certain polymers and organic materials, common in the production of medical devices, to ionizing radiation has been shown to cause some level of degradation to the polymer or organic material. The extent to which a polymer or organic material degrades is believed to be related to the dose of ionizing radiation absorbed. Thus, where a device is constructed of polymeric or organic materials, the applied radiation dose should be high enough to sterilize the device while concurrently being as low as possible in order to minimize the amount of device degradation that occurs. Where used for permanent and absorbable polymers and copolymers, typical, final packaged device sterilization is achieved with a dose of approximately 25 kGy.
Additionally, certain polymers, when exposed to ionizing radiation, undergo chain scission which may result in the formation of free radical(s) along the affected polymer chain. Free radicals of this type are generally known to exist in polymers for only brief periods of time after generation. The high energy of free radicals makes them unstable, rapidly reacting or recombining whenever possible. If the free radical combines with another free radical, and those free radicals are on differing polymer chains, crosslinking occurs and effectively increases molecular weight. If the free radical formed on the irradiated polymer chain combines with another element such as, but not limited to, oxygen, it may result in a degradation reaction and possibly a decrease in overall polymer molecular weight. In either case, the free radical reaction rate is typically very fast once the necessary conditions exist. Where the free radicals react with an oxygen molecule, reactive oxidative species (ROS) may be generated.
ROS are chemically reactive and biologically active oxygen-containing species such as superoxide, hydrogen peroxide, singlet oxygen, hydroxyl radical, hypochlorite, peroxynitrite, and perhydroxy radical, and combinations thereof. Further, ROS are highly reactive due to the presence of unpaired valence shell electrons.
In biology, ROS serve critical functions involving the immune response. For example, superoxide is naturally generated during “the respiratory burst” by activated neutrophils during phagocytosis of a microbe and is the mechanism used by the engulfing polymorphonuclear leukocytes (PMNs) in order to destroy bacteria. In light of this, current antibacterial drug therapies use ROS, particularly hydroxyl radicals, as the mechanism for bactericidal action (Kohanski et al., Cell, 130, 797-810 (2007)).
ROS are also active in cell signaling, including but not limited to stimulating cell proliferation, differentiation, migration, apoptosis, and angiogenesis (Klebanoff, Annals Internal Medicine, 93, 480-9 (1980)) (Turrens, Jrl Physiol, 552 (2), 335-44 (2003)) (Veal et al., Molecular Cell, 26, 1-14 (2007)). In particular, it has been shown that ROS even at relatively low concentrations (micro- to nanomolar) act as key cell signaling molecules to regulate a variety of biological processes such as angiogenesis, cell proliferation, and cell migration (Veal et al., Mol. Cell.; 26(1): 1-14 (2007)) (D'Autréaux et al., Nature Reviews Molecular Cell Biology, 8, 813-824 (2007)). ROS have also been shown to be influential in platelet activation (Krotz et al., Arterioscler Throm Vasc Biol; 24: 1988-96 (2004)). Involvement in these biological processes places ROS in the critical role of regulating numerous physiologic and pathologic states, including but not limited to some cancers, cardiovascular disease, chronic wounds, aging and neurodegeneration. For instance, use of ROS in clinical therapy has been demonstrated in photodynamic therapy (PDT) for cancer treatment (Dolmans et al., Nature Reviews Cancer, 3, 380-7 (2003)).
Higher level of ROS is known to inhibit cell proliferation and even induce cell apoptosis. Thus, one application of such ROS generation materials is to make medical devices, e.g. stent and balloons, to treat stenosis and restenosis in humoral ducts, including blood vessel, bile duct, esophagus and colon.
A stenosis is an abnormal narrowing in blood vessels or other ducts that is caused by uncontrolled proliferation and deposition of cells, extracellular matrix, lipids and other cellular contents. Thus, materials that release high levels of ROS can be used to inhibit such cellular proliferation and resolve the stenosis through the induction of apoptosis.
Restenosis refers to the recurrence of stenosis that follows the interventions that treat the original stenosis. Restenosis usually pertains to blood vessel that has become narrowed; received treatment to clear the blockage and subsequently become re-narrowed. Restenosis can occur following interventions such as percutaneous transluminal coronary angioplasty and stent treatments. These cardiovascular interventions induce unwanted proliferation of vascular smooth muscle cells (neointimal hyperplasia), which eventually leads to the re-narrowing of blood vessels. To prevent restenosis, drug-eluting stent (DES) was introduced into clinical cardiology at the beginning of the 2000s. Antiproliferative drugs, such as paclitaxel (an anti-cancer drug) and sirolimus (an immuno-suppressive drug), were coated on the surface of cardiovascular stent and released locally to the blood vessel wall. These drugs effectively inhibit vascular smooth muscle cell proliferation, and thus prevent in-stent neointimal hyperplasia and consequently restenosis.
It has been demonstrated that high level of ROS, particularly hydrogen peroxide, can effectively inhibit the proliferation of smooth muscle cells (Deshpande, N. N., et al., Mechanism of hydrogen peroxide-induced cell cycle arrest in vascular smooth muscle. Antioxid Redox Signal, 2002. 4(5): p. 845-54) and other cells (Li, M., et al., Hydrogen peroxide induces G2 cell cycle arrest and inhibits cell proliferation in osteoblasts, Anat Rec (Hoboken), 2009. 292(8): p. 1107-13) & (Chen, Q. and B. N. Ames, Senescence-like growth arrest induced by hydrogen peroxide in human diploid fibroblast F65 cells. Proc Natl Acad Sci USA, 1994. 91(10): p. 4130-4). ROS generating materials thus can be used to make medical devices, such as stent and balloons, which once deployed can locally deliver high level of ROS to prevent/treat restenosis.
To date, the benefits of ROS have been limited due to the short nature of their existence and difficulties in providing them at therapeutic levels and durations to desired treatment sites. It has surprisingly been found that stabilized free radicals can be formed in certain polymers and such free radicals can, in turn, generate ROS when exposed to an oxygen containing aqueous environment. Given the biological relevance of ROS, materials, devices and methods that enable the extended generation of ROS at a treatment site would be advantageous in the medical field and are contemplated herein.